The present invention relates to a structure of a package for a semiconductor device. More particularly, it relates to a package for a semiconductor device wherein improved seam welding is carried out by welding the sealing cap and the projecting part of the sealing frame.
For the purpose of protecting a semiconductor element from mechanical impact and the atmosphere, a package made of, for example, alumina ceramic, is provided.
FIG. 1 is a perspective view of a conventional package, and FIG. 2 is a schematic cross-sectional view of the package of FIG. 1 taken along the line A--A, a cap 7 and electrodes 8,8' being additional elements not shown in FIG. 1.
FIG. 3 is a partially enlarged schematic cross-sectional view illustrating a proposed package with seam welding of the sealing cap to the sealing frame, wherein the flashes 12 are not removed prior to fixing of the package substrate to sealing frame.
As shown in FIG. 1 and FIG. 2, a sealing frame 3 is provided on a surface of a package substrate 1 made of ceramic. In the inner side of the package substrate 1, a semiconductor element is fixed to the surface of the package substrate 1, and the semiconductor element is electrically connected to the outer lead 2 through leading wires 5 and inner conductive patterns 2'. After provision of the semiconductor element 4, the leading wires 5 and the inner conductive patterns 2' are provided as mentioned above, and the sealing frame 3, which is made of, for example, KOVAR and which is preliminarily fixed to the package substrate 2 by silver soldering, etc., is sealed with the sealing cap 7 (FIG. 2) made of, for example, KOVAR, by welding. Conventionally, the process of welding the sealing cap 7 to the sealing frame 3 is by parallel seam welding wherein, as shown in FIG. 2, conical electrodes 8,8' are rolled while contacting both edges of the sealing cap 7. Seam welding is carried out by using heat which is produced by flowing a current between electrodes 8 and 8' at the contact portion of the sealing cap 7 and the sealing frame 3. The sealing frame 3, which is welded with the sealing cap 7, is usually obtained by blanking or presswork. As shown in FIG. 3, the sealing frame 3 produced by blanking or presswork has droops 11 and flashes 12, the shape of the flash 12 being a characteristic one produced by blanking or presswork.
Conventionally, in the seam welding of the sealing cap 7 to the sealing frame 3, the seam welding is carried out so that the side of the frame 3 having the droop portion 11 is welded to the sealing cap 7. Moreover, prior to seam welding of the sealing cap 7 to the sealing frame 3, it is conventional to remove the flashes 12 from the opposite side of the sealing frame 3 to ensure proper sealing between the sealing frame 3 and the package substrate 1.
In such seam welding, since the upper surface of the sealing frame 3 has the droop portion 11, a gap is made between the edges of the sealing frame 3 and the sealing cap 7. Further, in the conventional seam welding, the contact of the sealing frame 3 with the sealing cap 7 is an entire surface contact. Therefore, since the heating value per unit area is decreased, it is difficult to increase the temperature to the required temperature for the welding and, thus, the weldability of the sealing frame 3 to the sealing cap 7 is lowered. On the other hand, when the applied electric power is increased to improve the weldability, the package substrate 1 sometimes cracks due to the heat transmitted from the sealing frame 3.
During development of the invention and as shown in FIG. 3, it was proposed to weld the side of the sealing frame 3 having the droop portion 11 to the sealing cap 7 and fix the opposite side of the sealing frame 3 having gap 13 and flashes 12 produced by blanking or presswork to the package substrate 1 without removal of the flashes 12. However, a problem in the sealing effect between the sealing frame 3 and the package substrate 1 was found to occur in this proposed package.